Intravascular imaging is often used to identify diagnostically significant characteristics of a vessel. For example, an intravascular imaging system may be used by a healthcare professional to help identify and locate blockages or lesions in a vessel. Common intravascular imaging systems include intravascular ultrasound (IVUS) systems as well as optical coherence tomography (OCT) systems.
IVUS systems include one or more ultrasound transducers emitting ultrasound energy based on received electrical signals and sending return electrical signals based on ultrasound energy reflected by various intravascular structures. In some instances, a console with a high-resolution display is able to display IVUS images in real-time. In this way, IVUS can be used to provide in-vivo visualization of the vascular structures and lumens, including the coronary artery lumen, coronary artery wall morphology, and devices, such as stents, at or near the surface of the coronary artery wall. IVUS imaging may be used to visualize diseased vessels, including coronary artery disease. In some instances, the ultrasound transducer(s) can operate at a relatively high frequency (e.g., 10 MHz-60 MHz, in some preferred embodiments, 40 MHz-60 MHz) and can be carried near a distal end of an IVUS catheter assembly. Some IVUS systems involve 360-degree visualization of the vessel (e.g., mechanically rotating the IVUS catheter assembly, steering IVUS signals from phased-array transducers, etc.).
Electrical signals received by the transducer can be in the form of image information and can be used to construct images. In some systems, analog image information can be digitized into vector form. An image can then be constructed from a series of vectors. For example, M vectors each comprising N data points can be used to construct an M×N two-dimensional image. In some systems, images of vascular structures of a patient can be generated and displayed real-time to provide in-vivo visualization of such structures.
The transducer typically produces analog signals and operates at a particular frequency. Generally, the resolution of the image information received increases with the operating frequency of the transducer and the frequency of data acquisition from the transducer. That is, high frequency images tend to have better resolution than low frequency images. However, data acquired at a high frequency often includes greater signal loss, and thus a lower signal to noise ratio (“SNR”) when compared to low frequency images because of losses associated with high frequency transmission. This can result in dark, hard-to-see images or very noisy images if the image intensity is amplified via increased gain. As a result, most intravascular imaging is performed at a relatively low frequency, sacrificing image resolution for an improved SNR.
In some systems, image information is processed to improve the SNR. Processing can include combining data such as averaging, envelope detection, and/or selecting various data points to eliminate, such as outlier elimination. However, each processing step takes time. For example, in some systems, envelope detection can require each vector to be passed through the envelope detector one-by-one, slowing down the imaging process. If the processing delay is too long, it can become impossible to generate a real-time display for in-vivo visualization of the vascular structures being imaged.